Negative Dispersion-Less Coupling Structure and Its Application for Realization of Extremely Small Negative Couplings in Waveguide Filters

Abstract

Negative coupling is widely used in microwave filter design, especially for filters with finite transmission zeros (TZs). Conventionally, negative coupling is realized through capacitive irises or probes. However, when the negative coupling is small, capacitive irises need to be very narrow, which presents significant challenges in manufacturing and tuning the filters. In this article, a negative dispersion-less coupling structure (NDLCS) is proposed based on a partial-height post for waveguide filters. Compared to capacitive irises, the NDLCS achieves a comparable effect while allowing the minimum dimension of coupling structures to increase from tens of micrometers to a few millimeters. The NDLCS also enables the installation of tuning screws, which significantly lowers both the manufacturing difficulty and the sensitivity to manufacturing tolerances. Two waveguide filters with narrow bandwidths are designed using the proposed NDLCS and compared to filters with conventional capacitive irises and dispersive coupling structures. The investigated and measured results demonstrate the advantages of the proposed NDLCS in manufacturing and tuning when realizing small negative coupling for waveguide filters.